1. Field of the Invention
The present invention relates to a servo track writer (STW) for writing a servo track signal to a disk medium in a magnetic disk drive, and also to a driving method for the servo track writer.
2. Description of the Related Art
With a reduction in size and an increase in recording density of a recent magnetic disk drive, a track pitch has been rapidly decreased and it is required to position a head with a high accuracy on the order of nanometers in writing a servo track signal to a disk medium. To meet this requirement, an improvement in head positioning accuracy is being pursued by increasing the accuracy of a position sensor and the rigidity of a drive motor. In the drive motor having a sliding portion, however, the effects of minute play and nonlinearity increase. It is therefore necessary to reduce or remove the effects generated from the drive motor itself and also to reduce the effect due to disturbance vibrations.
A servo track writer is a device for writing a servo track signal as a reference signal for use in positioning of a head in a magnetic disk drive to a disk medium. The servo track signal is written in the form of concentric circles at a submicron pitch. In the condition that no signal has yet been written on the disk medium and that a mechanical portion of the magnetic disk drive has been assembled, the disk drive is set on the servo track writer. An actuator (external actuator) of the servo track writer is operated to make an internal actuator of the magnetic disk drive follow, thereby positioning the head relative to the disk medium. In this condition, the servo track signal is written on the disk medium. Accordingly, the positioning accuracy of the external actuator in the servo track writer has a great effect on a servo track signal writing quality which determines the performance of the magnetic disk drive.
In a conventional servo track writer (STW), a voice coil motor (VCM) or a servo motor is used as a driving portion of the external actuator, and a rigid pin (push pin) is mounted on the front end of an arm fixed to an output shaft of the motor. The positioning of the head relative to the disk medium is carried out by placing the push pin in contact with a carriage (actuator arm) of the magnetic disk drive, applying a bias current to the internal actuator of the magnetic disk drive to press the carriage against the push pin, detecting a position of the arm of the STW, and feedback controlling the driving portion of the external actuator.
In recent years, a noncontact type STW and a driving method therefor have also been proposed. The noncontact type STW has a light emitting element and a photodetecting element both mounted on the arm of the STW, in place of the above-mentioned rigid pin. In this connection, a target is provided on the carriage of the magnetic disk drive. The head positioning is carried out by directing light from the light emitting element onto the target, receiving the light reflected on the target with the photodetecting element to thereby detect a relative displacement between the arm of the STW and the carriage of the magnetic disk drive, and controlling the driving of the internal actuator of the magnetic disk drive so as to cancel this relative displacement.
However, in the external actuator of the conventional STW, a ball bearing is used as a bearing at the driving portion. The ball bearing necessarily has minute play, causing a reduction in positioning accuracy. If a preload applied to the ball bearing is increased to reduce the play, friction torque increases to cause a deterioration in steady-state error or a reduction in responsiveness. Further, in a region where the amount of rotational movement is very small, the ball bearing exhibits a composite behavior inclusive of rolling, sliding, and elastic deformation. Accordingly, high nonlinearity and discontinuity appear in a control system, and stability cannot be ensured more as the control accuracy is tried to become higher.
In the case that a hydrostatic air bearing is used as the bearing in the external actuator, the problem of friction can be solved. In this case, however, the actuator itself becomes large in size, so that it cannot be applied to a small-sized magnetic disk drive and its associated servo track writer. Moreover, the rigidity of the bearing is reduced to cause a reduction in accuracy or vibration resistance characteristics due to elastic displacement.
A further problem is that the magnetic disk drive itself becomes a vibration source because of rotation of the disk medium, inducing vibrations of the carriage and a suspension in the magnetic disk drive. The external actuator of the conventional STW does not include any means for controlling and suppressing the vibrations of the carriage and the suspension, resulting in vibrations of the head to deteriorate the servo track signal writing quality. To improve the vibration resistance characteristics, increasing the inertia of the external actuator is desirable. However, an increase in tact time due to a reduction in responsiveness is invited, and the increase in inertia also causes a hindrance to a size reduction of the STW.
A still further problem is that when the natural vibration frequency of the internal actuator of the magnetic disk drive becomes equal to an integral multiple of the rotational speed of the disk medium, large vibrations due to resonance occur to deteriorate the servo track signal writing quality. In the conventional STW, the rotational speed of the disk medium is reduced in writing a servo track signal, thereby avoiding the occurrence of resonance.
However, the recent magnetic disk drive is of a contact start stop (CSS) type such that an air flow is generated by the rotation of the disk medium to act on a head slider and the head slider is kept flying by the balance of a positive pressure and a negative pressure owing to the air flow. Accordingly, an adjustable range of the rotational speed of the disk medium is narrow, and there is a case that the adjustment of the rotational speed may be impossible depending on the kinds of the disk drive. If the rotational speed of the disk medium is lowered to reduce the resonance, the time required for writing of a servo track signal becomes long, resulting in a hindrance to the productivity of the magnetic disk drive.
It is therefore an object of the present invention to provide a servo track writer and a driving method therefor which can improve a head positioning accuracy and positioning response speed in a disk drive.
It is another object of the present invention to provide a servo track writer and a driving method therefor which can reduce the vibrations of the internal actuator in the disk drive due to a disturbance and vibrations generated in the disk drive itself.
It is a further object of the present invention to provide a servo track writer and a driving method therefor which can adjust the resonant frequency of the internal actuator without changing the rotational speed of a disk medium, thereby reducing the resonance of the internal actuator depending on the rotational speed of the disk medium.
In accordance with an aspect of the present invention, there is provided a servo track writer for writing a servo track signal to a disk medium in a disk drive including said disk medium, a head for reading/writing information from/to said disk medium, and an internal actuator for moving said head across tracks formed on said disk medium, said internal actuator having a rotatable carriage for supporting said head, said servo track writer comprising an external actuator having an arm having substantially the same rotational axis as that of said carriage, a drive unit for rotating said arm, a solid deformation element provided in relation to said arm, and an effector mounted on said arm and adapted to come into contact with said carriage; an internal actuator control unit for controlling said internal actuator; an external actuator control unit for controlling said external actuator; a solid deformation element control unit for controlling said solid deformation element; and an arm position detecting device for detecting a position of said arm; said solid deformation element being divided into at least a first segment and a second segment, said first segment functioning as an actuator for minutely displacing said carriage, said second segment functioning as a sensor for detecting a position of said effector.
Preferably, said solid deformation element is selected from the group consisting of a piezoelectric element, piezoelectric resin, composite piezoelectric element, magnetostrictive element, and electrostrictive element. Preferably, said effector is configured by said solid deformation element. As modifications, said solid deformation element is interposed between said arm and said effector, or at least a part of said arm is configured by said solid deformation element.
Preferably, said solid deformation element comprises a tubular stack type solid deformation element divided into a plurality of first segments and a plurality of second segments alternately arranged in the circumferential direction, where said first segments function as an actuator, and said second segments function as a sensor. Alternatively, said solid deformation element comprises a platelike shear type solid deformation element having a polarization direction along the thickness thereof and divided into a pair of first segments and a second segment interposed between said pair of first segments, wherein said first segments function as an actuator, and said second segment functions as a sensor.
In accordance with another aspect of the present invention, there is provided a driving method for a servo track writer having an arm having substantially the same rotational axis as that of a carriage in a disk drive, a drive unit for rotating said arm, a solid deformation element provided in relation to said arm, and an effector mounted on said arm and adapted to come into contact with said carriage, said driving method comprising the steps of driving said carriage so that said carriage and said effector come into contact with each other; inputting an arm target position command to rotate said arm; detecting a position of said arm; feedback controlling the rotation of said arm so that said detected position of said arm coincides with an arm target position; inputting a carriage target position command to drive a first part of said solid deformation element and to minutely move said carriage through said effector; detecting a relative position of said effector to said arm by using a second part of said solid deformation element; detecting a position of said carriage according to said detected position of said arm and said detected relative position of said effector; and feedback controlling the driving of said first part of said solid deformation element so that said detected position of said carriage coincides with a carriage target position.
In this method, said arm is rotated so that said effector describes a spiral locus relative to a disk medium rotating at a constant speed; and said solid deformation element drives said effector so that said carriage describes a circular locus at each servo track position on said disk medium in synchronism with said spiral locus. Preferably, said driving method further comprises the steps of detecting vibrations of said carriage by using said second part of said solid deformation element; and driving said first part of said solid deformation element in a direction of canceling said detected vibrations of said carriage. As a modification, said driving method further comprises the steps of accumulating vibrations of said carriage as electric charge in said second part of said solid deformation element; and supplying said electric charge to a resonant circuit connected to said second part of said solid deformation element to consume said electric charge as Joule heat.
As another modification, said carriage is driven so that a contact force between said carriage and said effector is maintained constant, thereby reducing vibrations generated in said carriage. Preferably, said driving method further comprises the step of changing the contact force between said carriage and said effector to thereby change a resonant frequency of said carriage, thereby suppressing the resonance of said carriage.
The above and other objects, features and advantages of the present invention and the manner of realizing them will become more apparent, and the invention itself will best be understood from a study of the following description and appended claims with reference to the attached drawings showing some preferred embodiments of the invention.